The present invention relates to an image processing apparatus and method thereof, and more particularly, to an image processing apparatus and method thereof which utilizes plural colors of recording material such as toner and performs density control for each of the color recording material such as toner.
When color image formation is performed by an image processing apparatus employing the conventional electrophotographic printing method, along with changes in various conditions of image formation, e.g. operational environment or printing quantity, a level of image density outputted in response to an inputted image signal changes. Therefore, an output image does not always present an original color tonality represented by an inputted image signal.
To cope with the above situation, a method has been proposed in the conventional image processing apparatus where a toner image (hereinafter referred to as a patch) used for detecting density of each color is experimentally formed on a photosensitive drum or an intermediate transfer member in order to determine density reproducibility in image formation. Density of the patch is then automatically detected by a density sensor, feeding back the detection result of image formation conditions e.g., quantity of exposure, developing bias or the like, to perform density control so that the color image is formed in a color tonality consistent with the original image, thereby obtaining a desirable color image output.
In most cases, a conventional density sensor is embodied e.g. near an intermediate transfer member.
FIG. 6 is a drawing which shows an arrangement of a density sensor. Hereinafter, descriptions will be provided for the arrangement of the density sensor with reference to FIG. 6.
In FIG. 6, reference numeral 102 denotes a light-emission device such as an LED or the like; and 101, a photoreceptor such as a pin-type photodiode or the like. In this example, an infrared LED is utilized as a light-emission device 102, which irradiates an infrared ray on Y, M, C and Bk toner formed on an intermediate transfer member 105, such as those indicated by reference numeral 105A and 105B. The reflected light is then detected by the photoreceptor 101 and converted to an electric signal. The light-emission device 102 and photoreceptor 101, which constitute the density sensor, are arranged at different angles with respect to the intermediate transfer member 105 as shown in FIG. 6. The photoreceptor 101 is configured for measuring diffused reflected light from the patch. The foregoing structure provides an advantage in that density of all colors (Y, M, C and Bk) of toner can be detected by a pair of the light-emission device 102 and photoreceptor 101. However it also has a disadvantage in that the output characteristic of the density sensor for a Y, M and C toner patch is different from that of a Bk toner patch. For this reason, different processing sequences are necessary for Y, M and C toner and for Bk toner, in order to convert an output of the density sensor into a density value.
Besides the above described configuration of the density sensor, it is also possible to comprise three colors of light-emission devices respectively corresponding to each spectrum of Y, M and C toner and corresponding photoreceptors, to detect the density of the respective patch. Such configuration provides an advantage in that the output characteristics for all four types of toner are the same. Therefore, only one type of processing sequence is necessary for converting an output of the density sensor into a density value. On the other hand, since three pairs of light-emission devices and photoreceptors are necessary, the production cost as well as the size of an apparatus increases. For this reason, such configuration is rarely adopted as a density sensor of a color image processing apparatus.
As a series of printing processes according to the electrophotographic printing method is executed, sometimes the surface of the intermediate transfer member 105 in the image processing apparatus becomes rough, as caused by a cleaning unit, provided to remove remaining toner from the surface, or an edge of a transferring material e.g., a print sheet or the like, scraping the surface or contacting with the surface of the intermediate transfer member. Moreover, sometimes toner scattering inside the apparatus adheres on the surface of the intermediate transfer member 105. As a result, intensity of the reflected light from the density-measure patch detected by the density sensor deviates from that under the normal condition of an intermediate transfer member, thus normal density detection cannot be performed.
Furthermore, when the surface of the intermediate transfer member 105 is cleaned, all the remaining toner is not always completely removed, but accumulates gradually on the surface, causing a change in the color of the surface of the intermediate transfer member 105, thereby reducing the reflectivity thereof. As a result, the density detection result obtained by the density sensor is largely influenced by the reflectivity of the intermediate transfer member 105 on which toner still remains, and a measured density value changes as the apparatus is used for a long period of time.
FIG. 7 is a graph showing relationships of toner density and output of the density sensor, in the case where density of patches is measured at three positions A, B and C on the intermediate transfer member 105, at which each of the positions has a different reflectivity. According to FIG. 7, the reflectivity of the intermediate transfer member 105 decreases as the position moves from A to C. Note that in FIG. 7, a measurement result is shown with respect to M toner and Bk toner. Since the same measurement results are obtained for Y, M and C toner, M toner will represent the color toner herein and the results of Y and C toner are omitted.
As a contrast processing method employing the above described density sensor, the following method may be considered. When density detection is performed by the density sensor as described above, in order to cope with the problem in which a density detection result largley varies depending on the reflectivity of the surface of the intermediate transfer member 105 serving as a background, density with respect to Bk toner is measured at two positions: on the background and the toner patch, and a relative density is obtained. Hereinafter, the contrast processing method will be described.
Assuming that incident ray (I.sub.o1) is irradiated on a background (density Du) where a patch is not formed and reflected ray (I.sub.r1) is obtained. The relation is expressed by the following equation. EQU I.sub.r1 =I.sub.o1 .multidot.10.sup.-Du (1)
Next, assuming that when a patch having density Dp is formed on the background having density Du, incident ray (I.sub.o2) is irradiated and reflected ray (I.sub.r2) is obtained. The relation is expressed by the following equation. EQU I.sub.r2 =I.sub.o2 .multidot.10.sup.-(kDp+Du) (2)
Herein, k is a proportional coefficient depending on the type of toner and the configuration of the density sensor 2. The equation (2) is a theoretical equation expressing intensity of light detected by the density sensor 2 when light is irradiated on a toner (Bk toner) which shows an absorptive characteristic.
Herein, assuming that voltage values corresponding to the reflected light are V.sub.ref1 and V.sub.ref2 in equations (1) and (2) respectively, they are expressed by the following equations. EQU V.sub.ref1 =I.sub.o1 .multidot.10.sup.-Du (3) EQU V.sub.ref2 =I.sub.o2 .multidot.10.sup.-(kDp+Du) (4)
In order to calculate patch density Dp, the equation (3) is divided by equation (4) and the following equation (5) is obtained. EQU V.sub.ref1 /V.sub.ref2 =(I.sub.o1 /I.sub.o2).multidot.10.sup.kDp(5)
Accordingly, the patch density Dp is calculated by the following equation (6). EQU kDp=log(I.sub.o2 /I.sub.o1 .multidot.V.sub.ref1 /V.sub.ref2)(6)
According to equation (6), density of Bk toner is expressed by detected voltages corresponding to the incident ray and reflected light respectively obtained from the background and toner patch. Therefore, by respectively measuring density at two positions on the background and toner patch, and dividing one measurement result by the other measurement result, a relative density to the background patch can be obtained for Bk toner. Accordingly, it is possible to evaluate density of Bk toner with which background density is removed.
When a relative density value of color toner patch (Y, M and C) is to be calculated in the similar manner as the Bk toner according to the contrast processing method, a processing sequence different from that of the above described Bk toner patch is necessary. In this case, the following problem arises and density detection is difficult. Hereinafter, the problem will be described in detail.
Assuming that incident ray (I.sub.o1) is irradiated on a background (density Du) where a patch is not formed and reflected ray (I.sub.r1) is obtained. The relation is expressed by the following equation. EQU I.sub.r1 =I.sub.o1 .multidot.(1-10.sup.-Du) (7)
Herein, reflectivity of a background is expressed as Ru and defined by the following equation. EQU 10.sup.-RU =1-10.sup.-Du (8)
With the above, the following equation can be obtained. EQU I.sub.r1 =I.sub.o1 .multidot.10.sup.-Ru (9)
Next, assuming that when a patch having density Dp is formed on the background having density Du, incident ray (I.sub.o2) is irradiated and reflected ray (I.sub.r2) is obtained. The relation is expressed by the following equation. ##EQU1##
In order to evaluate the patch density Dp, as similar to the above described case of Bk toner using the contrast processing method, if equation (9) is divided by equation (10) assuming that voltage values corresponding to the reflected light are V.sub.ref1 and V.sub.ref2, a constant term Ru still remains. Thus, the patch density Dp cannot be expressed by the incident ray and detected voltage corresponding to the reflected light respectively obtained from the background and toner patch, as similar to the case of the Bk toner. In other words, the density (Dp) cannot be easily evaluated.
Therefore, it is necessary to obtain the patch density Dp and the background density Du from the equations (9) and (10) without performing the division, and obtain a relative density by subtracting the background Du from the patch density Dp. However in this case, incident ray intensity (Io) and a proportional coefficient (k) must be evaluated. Moreover, an absolute method of measuring the incident ray intensity (Io) is not established. Accordingly, there has been no appropriate method suggested for the contrast processing method which is utilized to measure relative density of Y, M, C color toners, leaving various problems to be solved.